The present invention generally relates to acceleration sensors (i.e., accelerometers) and, more particularly, relates to a microfabricated capacitively coupled linear accelerometer.
Accelerometers are commonly employed to measure the second derivative of displacement with respect to time. In particular, linear accelerometers measure linear acceleration along a particular sensing axis. Linear accelerometers are frequently employed to generate an output signal (e.g., voltage) proportional to linear acceleration for use in a vehicle control system. For example, the sensed output from a linear accelerometer may be used to control safety-related devices on an automotive vehicle, such as front and side impact air bags. In addition, low-g accelerometers are increasingly being used in automotive vehicles for vehicle dynamics control and suspension control applications.
Conventional linear accelerometers often employ an inertial mass suspended from a frame by multiple support beams. The mass, support beams, and frame generally act as a spring mass system, such that the displacement of the mass is proportional to the linear acceleration applied to the frame. The displacement of the mass generates a voltage proportional to linear acceleration, which is used as a measure of the linear acceleration.
One type of an accelerometer employs a capacitive coupling between a fixed plate and a movable plate that is movable in response to linear acceleration. For example, some capacitive type linear accelerometers employ an inertial mass suspended around the outer perimeter to a frame and having a movable capacitive plate separated from, and capacitively coupled to, a fixed capacitive plate such that displacement of the mass and movable plate changes the capacitive coupling between the fixed and movable plates. Prior known capacitive type linear accelerometers are configured with the mass connected to a substrate at the outer periphery. Such conventional acceleration sensors have a number of drawbacks which include susceptibility to poor sensitivity, fabrication processing complications, susceptibility to impulsive shocks due to handling, and problems caused by temperature-induced stresses.
Accordingly, conventional linear accelerometers often suffer from various drawbacks including deficiencies in sensitivity of the microsensor due to the structural asymmetries, fabrication processing, packaging, impulsive shocks due to handling, and temperature-induced stresses. It is therefore desirable to provide for a low cost, easy to make and use, and enhanced sensitivity linear accelerometer that eliminates or reduces the drawbacks of prior known linear accelerometers.
In accordance with the teachings of the present invention, a linear accelerometer is provided having a substrate, a fixed electrode supported on the substrate and including a first plurality of fixed capacitive plates, and an inertial mass substantially suspended over a cavity and including a plurality of movable capacitive plates arranged to provide a capacitive coupling with the first plurality of fixed capacitive plates. The inertial mass is linearly movable relative to the fixed electrode. A central member is fixed to the substrate and located substantially in a central region of the inertial mass. A plurality of support arms support the inertial mass relative to the fixed electrode and allow linear movement of the inertial mass upon experiencing a linear acceleration along a sensing axis, and prevent linear movement along a nonsensing axis. An input is electrically coupled to one of the fixed electrodes or the inertial mass for receiving an input signal, and an output is electrically coupled to the other of the fixed electrode or the inertial mass for providing an output signal which varies as a function of the capacitive coupling and is indicative of linear acceleration along the sensing axis.
By connecting the inertial mass to the fixed central member via the plurality of support arms, the linear accelerometer is less sensitive to stresses induced by fabrication processing, packaging, handling, and structural asymmetries. The realization of high mechanical sensing gain is also realized with the linear accelerometer to achieve enhanced immunity to electromagnetic interference (EMI) signals and environmental conditions, such as temperature. The linear accelerometer also provides high gain for linear accelerations about the sensing axis, while minimizing the effects of sensitivity due to linear off-axis accelerations and rotational cross-axis accelerations.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.